import java.util.*; import java.lang.*; import java.io.*; // The main method must be in a class named "Main". class Main { public static void main(String[] args) { Scanner sc = new Scanner(System.in); int n = sc.nextInt(); int d = sc.nextInt(); int[] t = new int[n]; int[] k = new int[n]; for (int i=0; i<n; i++) { t[i] = sc.nextInt(); k[i] = sc.nextInt(); } long t1 = t[0]; long k1 = k[0]-d; long tt = 0l; long kk = 0l; if(n <= 1){ tt = t1; } for (int i=1; i<n; i++) { tt = Math.max(t1 + t[i], k1 - d + t[i]); kk = Math.max(k1 + k[i], t1 - d + k[i]); t1 = tt; k1 = kk; } System.out.println(Math.max(tt,kk)); } }